1. Field of the Invention
The present invention relates to mechanisms for extracting water from a web of material, and, more particularly, from a fibrous web being processed into a paper product on a papermaking machine. Specifically, the present invention is a method for manufacturing resin-impregnated endless belt structures designed for use on a long nip press of the shoe type on a papermaking machine, and for other papermaking and paper-processing applications, and the belt structures manufactured in accordance with the method.
2. Description of the Prior Art
During the papermaking process, a fibrous web of cellulosic fibers is formed on a forming wire by depositing a fibrous slurry thereon in the forming section of a papermachine. A large amount of water is drained from the slurry in the forming section, after which the newly formed web is conducted to a press section. The press section includes a series of press nips, in which the fibrous web is subjected to compressive forces applied to remove water therefrom. The web finally is conducted to a drying section which includes heated dryer drums around which the web is directed. The heated dryer drums reduce the water content of the web to a desirable level through evaporation to yield a paper product.
Rising energy costs have made it increasingly desirable to remove as much water as possible from the web prior to its entering the dryer section. As the dryer drums are often heated from within by steam, costs associated with steam production can be substantial, especially when a large amount of water needs to be removed from the web.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long press nips of the shoe type has been found to be more advantageous than the use of nips formed by pairs of adjacent press rolls. This is because the longer the time a web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip which can be five to ten times longer in the machine direction than one formed between two press rolls is formed. Since the long nip is five to ten times longer than that in a conventional two-roll press, the so-called dwell time of the fibrous web in the long nip is correspondingly longer under the same level of pressure per square inch in pressing force used in a two-roll press. The result of this new long nip technology has been a dramatic increase in dewatering of the fibrous web in the long nip when compared to conventional nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in U.S. Pat. No. 5,238,537. This belt is designed to protect the press fabric supporting, carrying and dewatering the fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Belts of the variety shown in U.S. Pat. No. 5,238,537 are made by impregnating a woven base fabric, which takes the form of an endless loop, with a synthetic polymeric resin. Preferably, the resin forms a coating of some predetermined thickness on at least the inner surface of the belt, so that the yarns from which the base fabric is woven may be protected from direct contact with the arcuate pressure shoe component of the long nip press. It is specifically this coating which must have a smooth, impervious surface to slide readily over the lubricated shoe and to prevent any of the lubricating oil from penetrating the structure of the belt to contaminate the press fabric, or fabrics, and fibrous web.
The base fabric of the belt shown in U.S. Pat. No. 5,238,537 may be woven from monofilament yarns in a single- or multi-layer weave, and is woven so as to be sufficiently open to allow the impregnating material to totally impregnate the weave. This eliminates the possibility of any voids forming in the final belt. Such voids may allow the lubrication used between the belt and shoe to pass through the belt and contaminate the press fabric or fabrics and fibrous web. The base fabric may be flat-woven, and subsequently seamed into endless form, or woven endless in tubular form.
When the impregnating material is cured to a solid condition, it is primarily bound to the base fabric by a mechanical interlock, wherein the cured impregnating material surrounds the yarns of the base fabric. In addition, there may be some chemical bonding or adhesion between the cured impregnating material and the material of the yarns of the base fabric.
Long nip press belts, such as that shown in U.S. Pat. No. 5,238,537, depending on the size requirements of the long nip presses on which they are installed, have lengths from roughly 13 to 35 feet (approximately 4 to 11 meters), measured longitudinally around their endless-loop forms, and widths from roughly 100 to 450 inches (approximately 250 to 1125 centimeters), measured transversely across those forms.
It will be recognized that the length dimensions of the long nip press belts given above include those for belts for both open- and closed-loop presses. Long nip press belts for open-loop presses generally have lengths in the range from 25 to 35 feet (approximately 7.6 to 11 meters). The lengths (circumferences) of long nip press belts for some of the current closed-loop presses are set forth in the following table:
Belt Length (mm) Manufacturer Type Diameter (mm) (Circumf.) Valmet Symbelt Press 1425 4477 " 1795 5639 " 1995 6268 Voith Flex-O-Nip 1270 3990 " 1500 4712 Nip-Co-Flex 1270 3990 " 1500 4712 Intensa-S 1270 3990 " 1550 4869 Beloit ENP-C 1511 4748 (59.5 inch) " 2032 6384 (80 inch)
It will be appreciated that the manufacture of such belts is complicated by the requirement that the base fabric be endless prior to its impregnation with a synthetic polymeric resin.
Nevertheless, belts of this variety have been successfully manufactured for some years. However, two lingering problems remain in the manufacturing process.
Firstly, it remains difficult to remove all of the air from the base fabric during the impregnation and coating process. As implied above, air remaining in the woven structure of the base fabric manifests itself as voids in the final belt product. Such voids may allow the lubrication used between the belt and the arcuate pressure shoe to pass through the belt and contaminate the press fabric or fabrics and fibrous web. As a consequence, it is important to get all air out of the base fabric to achieve its complete impregnation by the synthetic polymeric resin being used.
Secondly, it remains difficult to provide the inner surface of the belt with a layer of synthetic polymeric resin without inverting the belt (turning it inside out) at some point during the manufacturing process. It will be appreciated that belts of the dimensions given above are not readily turned inside out, and that the act of doing so places a great strain on the impregnating and coating material, often leaving weak spots which may develop into full-fledged holes through the belt. Accordingly, the widely used technique of providing a layer of polymeric resin material on the outside of the belt, and inverting of the belt to place the layer on the inside, has not yielded consistently satisfactory results.
The present invention provides a solution to these problems, which characterize prior-art methods for manufacturing resin-impregnated endless belt structures, by including the use of an endless base fabric having a more open structure than those of the prior art to decrease the likelihood that air will be trapped therewithin, and by providing a layer of the polymeric resin material on the inner surface of the belt without having to turn the belt inside out at any time during the manufacturing process.